Apple discontinued development of Mac OS 9 in 2001, transitioning all future development to Mac OS X. Since that time, no updates have been released. The final updates to Mac OS 9 addressed compatibility issues with Mac OS X while running in the Classic Environment and compatibility with Carbon applications. At the 2002 World Wide Developers Conference, Steve Jobs began his keynote address by staging a mock funeral for OS 9.[4]

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Apple billed Mac OS 9 as including "50 New Features" and heavily marketed its Sherlock 2 software, which introduced a 'channels' feature for searching different online resources and introduced a QuickTime-like metallic appearance. Mac OS 9 also featured integrated support for Apple’s suite of Internet services known as iTools (later re-branded as .Mac, then MobileMe, which was replaced by iCloud) and included improved TCP/IP functionality with Open Transport 2.5.

PowerPC versions of OS X prior to 10.5 include a compatibility layer (a shell) called Classic, enabling users to run applications and hardware requiring Mac OS 9 from within OS X. This is achieved through running Mac OS 9 without access to Finder inside OS X. This requires Mac OS 9 to be installed on the computer, even if computers that can run the Classic environment are not necessarily able to boot into Mac OS 9. Some Mac OS 9 applications do not run well in Classic. They demonstrate screen redraw problems and lagging performance. In addition, some drivers and other software which directly interact with the hardware fail to work properly.

In May 2002, at Apple's Worldwide Developers Conference in San Jose, California, Steve Jobs, accompanied by a coffin, held a mock funeral[4] to announce that Apple had stopped development of Mac OS 9. Mac OS 9.2.2, released in December 2001, was the final version of Mac OS 9 and the "classic" Mac OS. In June 2005, Jobs announced that the Macintosh platform would be transitioning to Intel x86 microprocessors. Developer documentation of the Rosetta PowerPC emulation layer revealed that applications written for Mac OS 8 or 9 would not run on x86-based Macs. The Classic Environment remains in the PowerPC version of OS X 10.4; however, x86 versions of OS X do not support the Classic environment.

As a workaround for Intel-based Macs, Mac OS 9 can be emulated up to version 9.0.4 by using SheepShaver, a PowerPC emulator. It cannot emulate above 9.0.4 because SheepShaver does not emulate a memory management unit. The PearPC PowerPC emulator does not support Mac OS 9.[7]

While Mac OS 9 is no longer supported by Apple, retail copies are still available from several Internet businesses at varying prices.

Although now classed as abandonware, as development on it has ended, it is still in use by those who cannot upgrade to OS X due to hardware limitations, or prefer it to OS X. Mac OS 9 is also a popular choice for retrocomputing hobbyists. Mac gamers also sometimes revert to Classic (often via emulation) or native OS 9 in order to play games that aren't supported on OS X; for example: Bugdom, Nanosaur, Oregon Trail, Civilization II, and the Marathon Trilogy.

Aside from original Apple based hardware that is still maintained and operated, Mac OS 9 can be operated in other environments such as Windows and Unix.[8] For example, the aforementioned SheepShaver software was not designed for use on x86 platforms and required an actual PowerPC processor present in the machine it was running on similar to a hypervisor. Although it provides PowerPC processor support, it can only run up to Mac OS 9.0.4 because it does not emulate a memory management unit.

Updates to Mac OS 9 include 9.0.4, 9.1, 9.2.1, and 9.2.2. Mac OS 9.0.4 was a collection of bug fixes primarily relating to USB and FireWire support. Mac OS 9.1 included integrated CD burning support in the Macintosh Finder and added a new Window menu in the Finder for switching between open windows. Mac OS 9.2 increased performance noticeably and improved Classic Environment support. While Mac OS 9.2 is only officially supported on G3 Power Macs, it could still be installed on certain unsupported PowerPC models through the use of third-party software, such as Mac OS 9 Helper.[9]

1Some newest G3 and most G4 Macs can only run at least Mac OS 9.1, and some only run at least Mac OS 9.2. This is because the late development of G3 Macs and the mid-development of G4 Macs was during the development of Mac OS 9 and only the versions the G4 Macs support were designed to compatible with those G4 Macs.

1Some newest G4 and all G5 Macs can only run at least Mac OS 9.2, and can only run its compatible versions of Mac OS 9 in OS X's Classic Environment because the "Mac OS ROM" was never updated to allow those Macs, which were developed during the OS X era, to directly boot it (but probably continued to run in userspace, with the restriction that it would only run at least Mac OS 9.2, so it was still supported in the Classic Environment).